Method of operating a fuel cell system with integrated feedback control

ABSTRACT

A recirculating reagent fuel-cell includes an ion-exchange membrane interposed between an anode and cathode anode to form a membrane/electrode assembly (MEA), the MEA interposed between a fuel gas diffusion layer and an air (oxidant gas) diffusion layer. An air and fuel flow network are provided having an input portion for supplying reagent and an output portion for removing reagent after electrochemical reaction. At least one of the air flow network and fuel flow network includes a recirculation loop, the recirculation loop feeding back a portion of the fuel or air after electrochemical reaction to their respective input portion. The air flow network can include a water vapor condenser where water formed on the cathodes in proportion to the external load on the fuel cell stack is extracted and the fuel flow network can include an evaporator, where water is fed to the evaporator in the fuel feed loop from the condenser of the air feed loop.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuing application of copending application Ser. No.10/746,405, filed Dec. 24, 2003, which claimed the benefit, under 35U.S.C. 119(e), of provisional application No. 60/519,184, filed Nov. 12,2003; the prior applications are herewith incorporated by reference intheir entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to fuel cell assemblies and, more particularly tofuel cells having integrated feedback for regulation of water as well asfuel and oxidant supplied thereto.

Fuel cells hold great promise for commercial use in mobile andstationary power supply systems. Fuel cells electrochemically convertfuels and oxidants to electricity. Fuel cell types include Alkaline FuelCells (AFC), Molten Carbonate Fuel Cells (MCFC), Phosphoric Acid FuelCells (PAFC), Proton Exchange Membrane Fuel Cells (PEMFC or PEM), SolidOxide Fuel Cells (SOFC) and Direct Methanol Fuel Cells.

There has been significant progress in the development of fuel cells,including improvements in specific characteristics, such as increasedpower density and increased efficiency. Nonetheless, the wide variationsin load demand encountered in most commercial applications remain aproblem for fuel cell based electrochemical generators, particularly forthose that use solid polymer electrolytes, such as PEMs.

A PEM fuel cell converts the chemical energy of fuels such as hydrogenand an oxygen-containing gas (e.g. air) directly into electrical energy,water and heat. At the heart of a PEM fuel cell is a membrane electrodeassembly (MEA) comprised of a proton conducting membrane electrolytesandwiched between two gas diffusion electrodes. The membrane permitsthe passage of protons (H+) generated by the oxidation of hydrogen gasat the anode to reach the cathode side of the fuel cell and form water,while preventing passage therethrough of either of the reactant gases.

Efficient operation of PEM fuel cells generally requires the removal ofa portion of the water produced. Excess water can feel up the pores ofthe gas diffusion layers effectively cutting of the gases from membraneand stopping the chemical reaction. Load demands faced by a system in atypical commercial use might vary from 0 to 1000 mA/cm² under a typicalload cycle.

For the optimum operation of such fuel cells, the membrane should remainsufficiently moist throughout, but not too moist. Thus, there must beremoval of a portion of the water generated at the cathode, as well asthe addition of water at the anode side to provide sufficient membranemoistness.

Several characteristics of PEM fuel cells separate them from other typesof fuel cells. For example, in contrast to other fuel cell types, PEMfuel cells have a narrow range for controlling optimal concentration ofelectrolyte in the localized zone of electrochemical activity comprisingthe anode, membrane and cathode. Such membranes have a limited abilityfor redistribution of water over the fuel cell working surface area.This performance characteristic of fuel cells with PEMs is attributed tothe reduced ability of the anode, cathode and membrane (as a group) totransport water, and to the hydrophobic characteristics of the materialsused.

These characteristics of solid-polymer membranes become critical whendesigning and using fuel cells with large working surface areas toproduce large currents, such as required for transportation applications(e.g. automobiles, and busses) especially when a large number of fuelcells are combined in series to generate high voltage outputs. Forexample, to build an electrochemical generator having a capacity of 25kW at a voltage of 120 V, a stack comprising 160 fuel cells is requiredwith a working surface area of approximately 600 cm² each. In agenerator with a power rating of 60 kW and a 330 V output, it isnecessary to install 420 fuel cell elements with working surface area of740 cm² each, connected in series.

Maintaining the high output characteristics of fuel cells assembled intostacks to form electrochemical generators is one of the challenges ofelectrochemical generator design. In the case of fuel cells withsolid-polymer membranes this task is even more difficult. The verynarrow range over which water concentration must be controlled imposesstrict requirements on the systems that feed the working gases, as wellas on regulation of water concentration and temperature of eachindividual fuel cell. In addition, even at low operating times(1000-2000 hrs), characteristics of the individual fuel cells in a stackdo not change in a constant or even manner. Progressive and unevendegradation in performance among the cells demands even more strictrequirements for control of fuel cells assembled into electrochemicalgenerator systems.

In high power hydrogen-air electrochemical generators, hydrogen issupplied from storage tanks with high pressures up to 70 MPa. Systemsfor supplying gas usually have electric valves on hydrogen supply andpurge lines. A hydrogen pressure regulator is commonly installed in thegas supply line upstream of the fuel cell stack. A feedback controlpressure regulator is generally provided which senses variation inpressure at the fuel cell and control reactants gas flow in a mannerproportional to gas usage. Control of gas flow and pressure (i.e.reduction of pressure from input pressure to working pressure) is alsoaccomplished using a regulator.

For smoother and more precise throttle control, a two-stage pressureregulator system is usually installed. The pressure regulator reducesthe working pressure of the fuel cell. For synchronization of hydrogenand air pressures in the fuel cell stack, a pressure reference line isinstalled in parallel to hydrogen supply line to provide a referencepressure to the regulator.

This reference line is static and does not consume hydrogen during fuelcell operation. It is filled with hydrogen during start-up and emptied(purged) when the fuel cell generator is stopped or stored. As a rule, avent valve is installed in the reference line to restrict pressure, andan electrical valve is installed for reduction of pressure toatmospheric pressure.

The reference line can be filled with inert gas, if available. Theoxidant feed line to the cathode pores in the fuel cell stack has afilter to remove particles and a compressor to built up air pressure toa working level. The partial pressure of oxygen in air is relatively low(about 21%), the largest portion of air being nitrogen. For the cathodeto work effectively, air should be fed in excess. In this case, theefficiency of oxygen usage is 40%-60% as a rule. At higher rates ofoxygen usage, the cathode is less efficient.

In current fuel cell stack designs, the air supply system maintains thedesign working pressure level on cathode and anode. For this purpose,the hydrogen pressure regulator has a feedback connection to the airsupply line at the entry point to the fuel cell. In this case thehydrogen pressure in the anode chamber is constantly compared with theair pressure in the cathode chamber and the pressure regulator makesneeded adjustments in order to maintain the correct pressure ratio.

The system described above for supplying hydrogen and air to fuel cellswith solid-polymer electrolytes is essentially universal and used inalmost all known designs with only minor variations. However, asexplained below, these systems do not provide good regulation of thewater concentration along the cathode and anode surface of the fuel cellstack, particularly for high and highly variable load conditions.

The power output of a hydrogen-air fuel cell mainly depends on effectiveperformance of the cathodes (oxygen limited electrodes).

In this case, there are gas transport restrictions on the amount ofoxygen penetrating through the cathode pores and available to thecathodes. Drying takes place in some areas of the cathodes because oflow water (vapor) concentration in the air supplied by the compressor.

Moreover, compressed feed air at the outlet of the compressor can be aneven higher temperatures (e.g. 110-150° C.). Thus, there is activeremoval of water (vapor) by the airflow which, in turn, leads to dryingof the membrane in the air inlet region. In the air outlet area from thecathodes there occurs the reverse of this process leading to “flooding”of the cathode because air flowing in this area is close to saturationby water vapors and the rate of water uptake (vaporization) is lower.

Because of low oxygen concentrations in the air after passing through,most of the cathode chamber and gas flow restrictions, a large portionof the cathode surface can be in a condition of “concentrationpolarization.” Concentration polarization results from restrictions tothe transport of the reactant gases to the reaction sites. This usuallyoccurs at high current because the forming of product water and excesshumidification blocks the reaction sites. In this situation, there isincreased risk of cross polarization in the area near the gas outletfrom the cathode chamber. This risk becomes much greater when the fuelcell load is highly variable over short time periods. Specifically, therisk is greatest when loads are switched from low to high levels andback in short periods of time, such as tens of seconds to minutes.

Such short-term load variations are generally not allowed in fuel celloperation. Otherwise, non-optimum humidity can lead to crosspolarization. This can cause the cells to operate in an electrolysismode, which in turn can lead to direct reaction between hydrogen and airin the cell resulting in physical damage to the fuel cell.

Solving the problem of controlling in fuel cells will greatly expandpotential of their application. However, this does not solve the problemof the fuel cell's inability to withstand wide range, short-termvariations in load because of high thermal inertia due to the heatcapacity of the fuel cell stack. The primary unmet requirement for useof hydrogen-air fuel cells in transportation and many stationary powerapplications is that fuel cell generators must be highly reliable in theface of rapid and wide-range variations in load.

The above-mentioned issues represent a significant problem forelectrochemical generators with solid polymer fuel cells as presentlyinstalled on electric vehicle prototypes. Currently availableelectrochemical generators do not meet consumer requirements in thisregard, and therefore cannot be mass-produced and marketed for generaluse. This is not only because of the high cost and complexity of systemsfor controlling processes in fuel cells, but also because a primaryapplication requirement cannot be met. This requirement is the abilityto handle current loads that vary widely, and sometimes rapidly, forlong-term operation (e.g. more than about 3000 hrs.).

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a fuel cellsystem and method which overcomes the above-mentioned disadvantages ofthe heretofore-known devices and methods of this general type.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a method of operating a PEM fuel cell,which comprises the following steps:

-   -   providing a fuel flow to an anode side of the fuel cell;    -   providing an air flow to a cathode side of the fuel cell;    -   recirculating a portion of the air flow, after reaction thereof        at the anode side, from an output to an input of the cathode        side; and    -   selectively pressurizing the fuel flow and the air flow, with a        length of a pulse period and a duty cycle of increased pressure        within the pulse period adjusted to an instantaneous power        requirement of the fuel cell.

A recirculating reagent fuel-cell includes an ion-exchange membraneinterposed between an anode and cathode to form a membrane/electrodeassembly (MEA), the MEA interposed between a fuel gas diffusion layerand an oxidant gas diffusion layer. An oxidant and fuel flow network areprovided having an input portion for supplying reagent and an outputportion for removing reagent and reaction products after theelectrochemical reaction. At least one of the oxidant flow network andfuel flow network includes a recirculation loop formed by a feedbackconduit which provides fluid connection between the input and outputportion. The recirculation loop feeds back a portion of the fuel oroxidant after electrochemical reaction to their respective inputportion.

The recirculation loop can include a water containing volume, wherein aportion of the output flow flows through the water containing volume togenerate a humidified flow, the humidified flow comprising a portion ofthe oxidant or the fuel flow supplied to the fuel cell. The volume ofthe humidified flow can be adjustable, with the humidified flow volumeincreasing when a load on the fuel cell increases.

At least one of the oxidant and fuel input portions can include a jetpump therein, where the jet pump induces recirculation in therecirculation loop. The output flow of the feedback conduit ispreferably used as an input flow to the suction input of the jet pump.In this embodiment, the jet pump mixes the portion of the fuel oroxidant flow fed back with externally supplied fuel or oxidant.

The water containing volume in the oxidant flow network can be acondenser for extracting water from the cathode, while the watercontaining volume in the fuel flow network can be an evaporator. In thisembodiment, the condenser extracts water from the cathode in the amountdepending on a load on the fuel cell. The condenser is preferablyfluidly connected to the evaporator, with the condenser supplying thefuel flow network with water.

The fuel cell can include a fuel flow modulator fluidically connectedwith at least one of an input portion of the fuel flow network and aninput portion of the oxidant flow network, wherein the fuel flowmodulator provides a time varying mass flow of fuel and oxidant. Themodulator preferably includes structure for initiating operation acrossall fuel cell load conditions. The fuel flow network can include a fuelflow modulator and the oxidant flow network can include an oxidant flowmodulator, the first modulator being communicably connected with secondmodulator and controlling operations of second modulator. The flowmodulator preferably provides discrete pulses of fuel and oxidant flow,such as through use in the fuel flow network of a pressuresensor-controlled two-positional pressure regulator having only twopositions, a first position being a fully open and the other positionbeing fully closed and through use in oxidant flow network of apressure-sensing-two-chambers controlled differential pressureregulator.

A method of operating a fuel cell includes the steps of providing a fuelflow to an anode of the fuel cell and an oxidant flow to a cathode ofthe fuel cell, wherein at least one of the fuel flow and the oxidantflow comprises a recirculated flow portion. The recirculated flowportion can be a humidified flow. The fuel flow and the oxidant flow caninclude a recirculated flow portion, wherein the method can include thestep of transferring water generated at the cathode into the fuelrecirculated portion to humidify the fuel flow.

At least one of the fuel flow and the oxidant flow can be a time varyingmass flow, the mass flow varying with a load on the fuel cell. The timevarying mass flow is preferably operative across all loads on the fuelcell and can comprise discrete pressure pulses. In a preferredembodiment, both the fuel flow and the oxidant flow are time varyingmass flows, wherein the method can further comprise the step ofsynchronizing the time varying mass flow of the fuel flow with the timevarying mass flow of the oxidant flow.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

The invention is not intended to be limited to the details shown, sincevarious modifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of a recirculating reagent fuel cell system havingrecirculation loops in both the anode and cathode side, according to anembodiment of the invention;

FIG. 2 shows the various components of an exemplary jet pump;

FIG. 3 is a schematic model showing elements of an exemplary regulatedgas supply system comprising a closed vessel with variable gas inflow,consumption and outlet flow;

FIGS. 4A, 4B, and 4C show examples of gas supply periods, pauses andcycles of an aperiodic load based reagent flow supply arrangement underrelatively high, intermediate and low external load conditions,respectively, according to a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is an electrochemical generator based on fuel cells, suchas hydrogen-air fuel cells with solid polymer proton exchange membranes(PEM) that can be used in mobile or stationary applications. Generatorsbased on the invention provide higher reliability and higher efficiencyas compared to conventional fuel cells, particularly under rapid andwidely varying power demands, such as those encountered for typicalautomotive applications.

A recirculating reagent fuel-cell includes an ion-exchange membraneinterposed between an anode and cathode anode to form amembrane/electrode assembly (MEA), the MEA interposed between a fuel gasdiffusion layer and an oxidant gas diffusion layer. An oxidant and fuelflow network are provided having an input portion for supplying reagentand an output portion for removing excess reagent and reactionbyproducts after electrochemical reaction. At least one of the oxidantflow network and fuel flow network includes a feedback conduit to form arecirculation loop, the recirculation loop feeding back a portion of thefuel and/or oxidant after electrochemical reaction to their respectiveinput portion.

The oxidant flow loop can include a water vapor condenser to extractwater from the cathode chambers, the amount of water being based on theexternal load on the fuel cell stack. The fuel flow network can includean evaporator, where water is fed to the evaporator in the fuel loopfrom the condenser in the oxidant loop. In this embodiment, the portionof the output flow fed back to the input portion is a humidified flow.

The invention provides humidification and resulting membrane wetnesswhich is based on the fuel cell load. If the load increases, the fuelcell generates more water, thus more water is collected in thecondenser. Since the output flow portion flowing through the condenserincreases as the load increases, the humidified flow output by thecondenser increases as well based on the level of the load.

Although the invention is generally described with respect to ahydrogen-air electrochemical generator, the invention is in no waylimited to either hydrogen or air. For example, the fuel cangenerally-be any oxidizable gas, including mixtures thereof, while aircan more generally be any oxidant gas. Moreover, recirculating reagentgas flow arrangements according to the invention described herein can beadvantageously used with other types of fuel cells, particularly formembrane-based fuel cells. In addition, the aperiodic load based reagentflow supply feature described herein can be generally used with allfuels cell types, whether membrane based or not, and more generally, forchemically reactive systems.

Referring now to the figures of the drawing in detail and first,particularly, to FIG. 1 thereof, there is shown a schematic of arecirculating reagent fuel cell system 100 according to an embodiment ofthe invention is shown. System 100 includes fuel cell 5, which includesion-exchange membrane 29 interposed between an anode 31 and cathode 27to form a membrane/electrode assembly (MEA). The MEA is interposedbetween porous oxidant gas diffusion layer 26 and porous fuel diffusionlayer 32. Cathode chamber 28 is bounded by plate 24 which is disposedadjacent to oxidant gas diffusion layer 26, while anode chamber 38bounded by flow plate 34 is disposed adjacent to fuel diffusion layer32. The respective porous gas diffusion layer/electrode structurestypically comprise a Pt electrocatalyst dispersed on high surface areacarbon black, held together with a binding agents, such aspolytetrafluoroethyene (PTFE, Teflon®). In most practical electricalchemical generator applications, system 100 comprises a plurality offuel cells 5 hooked in series to form a fuel cell stack. The fuel cell 5arrangement described herein is not an aspect of the invention.

The reagent recirculation and control arrangement shown in both thecathode side 1 and anode side 2 are aspects of the invention. Cathodeside 1 is provided an air supply, preferably cleaned of particles bysuitable filtration, which is fed into a compressor 10, which providesthe necessary flow and pressure of oxidant (e.g. air) for cathode side 1of fuel cell 5 to support the electrochemical reaction. Both an electricmotor 12 and an expander 11 are preferably used to drive compressor 10.Expander 11 utilizes energy from a hot pressurized oxidant output flowafter electrochemical reaction.

Compressor 10 is in fluid communication with pressure regulator 25 vialine 48. The regulator 25 is preferably of the type“pressure-sensing-two-chambers-controlled differential pressureregulator”. This preferred type of regulator provides discrete pressurepulses of gas flow where a timing of these pulses is synchronized withpulse timing of the regulator 75 and flow volume through this regulatordepends on the external load and the gas consumption of theelectrochemical reaction, which is generally variable over time, and maybe highly variable. Regulator 25 senses pressure in the output portionof the oxidant flow network and is communicably connected to regulator75 on the anode side 2.

When the fuel cell 5 is operating in an idling mode, with external loadsdisconnected, compressor 10 in the cathode side 1 and the compressor (ifpresent) in the anode side 2 is preferably left running. This conditionallows fast re-connection to external load, because when fuel cells areoperated at the lower loads, the process of hydrogen and oxygen supplydoes not stop and can be rapidly increased as needed after re-connectingthe external load.

To increase the supply of oxidant gas to the cathode side of fuel cell 5without the need for additional air intake into system 100, and forextraction of water and depleted oxidant, an oxidant recirculationfeedback loop 15 is provided. Recirculation loop 15 comprises pump 50which is used to induce oxidant flow through cathode chamber 28, flowsplitter 20, and water vapor condenser 30 and associated connectinglines. In the arrangement shown in FIG. 1, condenser 30 along with itsassociated lines provides the feedback conduit between input portion (atpump 50) and the output portion (at flow splitter 20) of recirculationloop 15. Although shown in the feedback conduit in FIG. 1, condenser 30can be disposed between cathode chamber 28 and flow splitter 20.

After passing regulator 25, pressurized oxidant comprising gas is fedinto the inducing nozzle 51 of pump 50 at a typical pressure of 0.2-0.45MPa. Gas pump 50 is preferably a jet pump. For recirculation of bothfuel in anode side 2 and oxidant in cathode side 1, jet pumps arepreferred because they provide substantially proportional relationbetween consumption of recirculation streams and used gases in the fuelcells during the current production. Additional positive characteristicsof such pumps as compared to electromechanical pumps include highreliability, and essentially unlimited time in operation with no needfor electrical energy use. Jet pump 50 can be driven entirely bypotential energy of the compressed oxidant (e.g. stored in reagenttanks). Although jet pumps are preferred, other pump types may be usedwith the invention.

Now referring to FIG. 2, jet pump 50 is shown including variouscomponents designed to control pressure/flow characteristics. Theseinclude the high-speed gas ejection nozzle 51, a stream mixing chamber52 with diffuser 53 and a receiving chamber 54 for further gas mixing.

Gas passing through nozzle 51 forms a high-velocity stream in thereceiving chamber 54. This high-speed stream generates a lower pressureregion at its boundary (according to the Bernoulli principle) andthereby sucks in gas from receiving chamber 54. The two streams of airare directed into the mixing chamber 52 where their speed is equalizeddue to the mixing. The mixed stream then passes through a diffuser 53,where the stream is expanded, and the static pressure increases.

The coefficient of injection characterizes the ratio between the massflow of moistened air fed to the receiving chamber 54 of the jet pump 50and the airflow from compressor 10 to nozzle 51. The degree ofcompression of the mixed airflow output by pump 50 corresponds toaerodynamic resistance of the recirculation loop 15.

Throttling of the air stream occurs by passing the oxidant streamthrough the valve nozzle 51 of jet pump 50. The pressure regulator 25then enables stabilizing amount of oxidant gas going through the jetpump 50 in the face of arbitrary changes in oxidant consumption in thefuel cell stack. The optimal upper and lower levels of oxidant (e.g.air) pressure on the cathode can be selected for each specific type ofporous media.

Returning again to FIG. 1, after passing pump 50, the oxidant flow isthrottled and the pressure preferably drops to between about 0.02-0.05MPa according to the pressure in the circuit. Heat generated by the fuelcell 100 is shown extracted by an independent coolant loop designated as61 in FIG. 1. A portion of the oxidant, with depleted oxygenconcentration after electrochemical reaction, is directed from an outputportion of the recirculation loop 15 into a flow splitter 20, such as ableed air tee. Flow splitter 20 directs a specific portion or amount ofbleed oxidant following electrochemical reaction to expander 11 to usethe energy of this flow to help drive the compressor 10 along with maindrive motor 12, with the remaining depleted oxygen flow going tocondenser 30. Following energy extraction at expander 11, the depletedoxygen flow can be exhausted to the atmosphere.

Now turning to anode side 2 of the system 100, anode side 2 providesfuel, such as hydrogen along with humidification to anode 31 of fuelcell 5. Anode side 2 is provided a suitable source of hydrogen or otherfuel, preferably being a filtered source, such as from a pressurevessel. Hydrogen supplied first reaches solenoid valve 74 and thenpressure regulator 75. Regulator 75 is connected by piping to a pump 55,such as a jet pump having nozzle 57, which acts as to induce hydrogenflow in the closed recirculation loop 60. Hydrogen recirculation loop 60includes pump 55, anode chamber 38, hydrogen evaporator/humidifier 80,and associated tubing. The hydrogen recirculation loop 60 is a part ofthe fuel and water vapor supply system for the anode 31.

According to a preferred embodiment of the invention, the anode chamber38 of fuel cell 30 has channels in the hydrogen feed stream that directthe hydrogen flow in such way so as to distribute it uniformly over theanode operating surface. Such distribution is preferably optimized fordifferent anode sizes and geometrical forms.

As noted above, regulator 75 is communicably connected to regulator 25cathode side 1. The connection of regulators 75 and 25 can be preferablyvia a pneumatic line. The controlling set point of the regulator 75 isused as a reference point for the regulator 25. Such a connectionbetween fuel regulator 75 and air regulator 25 provides synchronizationof their operation.

Two-sided and simultaneous (relative to the polymer membrane 29 in fuelcell 5) control of pressure on anode 31 and cathode 27 is important inthe operation of the anode 31, membrane 29, and cathode 27 as a group.This arrangement improves the dynamic performance of fuel cell 5 duringload variations and also decreases the degradation rate of volt-amperecharacteristics of the fuel cell stack, due to the active anode andcathode ventilation to remove inert and contaminating gases and providefor more uniform distribution of water.

Pump 55 is shown as a jet pump as well as a pump 50 described withrespect to cathode side 1, while regulator 75 is preferably the“pressure sensor controlled two-positional pressure regulator” type. Jetpump 55 receives hydrogen supplied via regulator 75 (when open) which isprovided to nozzle 57. Pump 55 mixes hydrogen supplied by regulator 75(when open) with recirculated humidified hydrogen flow provided byevaporator 80. The mixed hydrogen stream emerges from pump 55 andreaches anode 31 of fuel cell 5. Regulator 75 preferably senses pressurealong an output portion 84 of the fuel recirculation loop 60.

At the hydrogen flow outlet of the fuel cell 5 at T-point 84, a purgeline for the anode chamber 38 is preferably connected with a throttle 87to restrict hydrogen flow when solenoid valve 88 is fully open.

FIG. 3 shows a schematic model of elements of an exemplary regulated gassupply system comprising a closed vessel with variable gas consumptionoutflow and compensating inlet flow. As noted above, pressure regulator75 is preferably of the type “pressure sensor-controlled two-positionalpressure regulator”. System 300 is a model for gas supply using such aregulator to a fuel cell with variable consumption in response to thespeed of the electrochemical reaction.

A gas (pressurized air for example) from a source 310 is modeled ashaving a mass flow which exceeds a mass flow of the consumption. Forexample, the pressure provided P_(I)=0.5 Mpa can be introduced into thevessel 390 via pipe 330 which has a two-position pressure regulator 391including two solenoids, namely 399 to open and 398 to close. Assumethat pressure in the vessel 390 is desired to be maintained at a stablelevel, such as P_(work)=0.3±0.03 MPa.

A throttle 392 is installed between pressure regulator 391 and thevessel 390 for restriction of gas flow. Gas flows through pipe 320 whichhas a throttle 393 to restrict exiting gas flow and a regulated throttle394, which reduces gas flow in pipe 320. On vessel 390, pressure sensors395 and 397 are installed with different pressure regulating parametersto operate solenoids 399 and 398, respectively.

Design of the two-position pressure regulator 391 allows only twoextreme positions of the valve and saddle, “fully open” and “fullyclosed.” Any intermediate positions of the valve relative to the saddleare not possible. Throttling of the gas stream entering the vessel 390occurs only at the throttle 392. Gas can exit the vessel only throughthe pipe 320 at a variable flow rate controlled by the flow areachanging of the regulating throttle 394. Maximum consumption of theeffluent gas through the pipe 320 is limited by the flow area of theunregulated throttle 393. It is assumed that the maximum gas inflow rateto the vessel through the pipe 330 is 1.5 times the maximum gasconsumption from the vessel through the pipe 320.

The object of system 300 is to control of the pressure in the vessel 390under conditions of variable gas effluent rates from the vessel. Twopressure sensors 395 and 397 are installed on the vessel 390. If thepressure has dropped to the some pre-determined level (for instance,P_(work)=0.270 MPa) the first pressure sensor 395 will command thepressure regulator 391 to open. If the pressure has reached somepre-determined level (for instance, P_(work)=0.330 MPa) the secondpressure sensor 397 will command the pressure regulator 391 to close. Asa result, system 300 delivers discrete pulses of gas at a constantpressure to vessel 390.

In conventional solutions to this problem, a “balanced-type” pressureregulator controls the gas supply to the fuel cell. The “balanced-type”pressure regulator in such a circuit has a measuring space directlyafter the valve saddle and throttling of the gas occurs in the gapbetween the valve and saddle. Such regulators can replace both pressuresensors 395 and 397 and the two-position regulator 391.

FIGS. 4A, 4B, and 4C show exemplary gas supply periods, pauses andcycles of an aperiodic load based reagent flow supply system underrelatively high, intermediate and low external load conditions,respectively, according to a preferred embodiment of the invention for afixed period of time, T_(I). Pop is the operating pressure, Pmax is themaximum operating pressure, Pmin is the minimum operating pressure, Pnomis the nominal operating pressure, T is the time, M_(R) is the masscirculation flow, Ts is the hydrogen supply time, Tc is the cycle timeand T_(p) is the pause time. To implement pauses and cycles of anaperiodic load based reagent flow a relay-type of pressure regulator canbe used. This preferred regulator has two positions, fully open andfully closed. In this preferred embodiment, a pressure-sensor controlledtwo-positional pressure regulator 75, or arrangement which providesequivalent flow dynamics responsive to system dynamics.

FIG. 4A shows the gas supply period, pauses and cycles under relativelyhigh load conditions. Under the high load conditions, the cycle time(Tc) which comprises a supply time (Ts) plus the pause time (Tp)provides a little over two (2) periods in the time T_(I). The supplytime (Ts) is nearly equal to the cycle time (Tc). When the regulator isopen the operating pressure (Pop) rises as a function of time until thetime when Pop reaches Pmax, then the regulator shuts off. While theregulator is off, the operating pressure decreases until P_(Min) isreached, and the regulator is turned on again. FIG. 4B shows the gassupply period, pauses and cycles under moderate load conditions.

Compiling the data from FIGS. 4A-4C, the supply time Ts increases as theload increases. In addition, the mass recirculation flow M_(R) increaseswith increasing load.

Thus, the preferred pressure-sensor controlled two-positional pressureregulator” can be characterized as a supply of gas pressure pulsationand as a supply of a pulsation of recirculating mass flow where thepulse dynamics change as a function of load. A difference between thereactant flow characteristics obtained using the preferred pressureregulator as disclosed herein as compared to pulsed reactant systemssuch as disclosed in U.S. Pat. No. 6,093,502 to Carlstrom, Jr. et al. isthe simultaneous variation of pulse width and pulse period to extenddepending on the external load and gas consumption rate of theelectrochemical reaction provided by the invention. In addition,Carlstrom's pulsed system is only activated upon detection of apredetermined high load level, while the pulsed gas supply of theinvention is preferably operable over all load conditions.

Again returning to FIG. 1, assuming regulator 75 is the type“pressure-sensor controlled two-positional solenoid valve,” or a devicewhich provides an equivalent response, which turns on when the pressureat 84 drops to P_(Min), and turns off when the pressure at 84 reachesPmax. When regulator 75 is fully open, gas flows through, such as intothe input portion of the recirculation loop 60 through pump 55, thusraising the operating pressure in loop 60. When regulator 75 is fullyclosed, thus pausing the gas supply provided to loop 60, then pressurein the loop 60 begins dropping until Pmin is reached, this pressurevalue is sensed, and as a result regulator 75 again turns on and a newcycle is initiated. In its fully closed position, the pressure upstreamfrom jet pump nozzle 57 is reduced synchronously with the pressure inthe recirculation loop 60, because gas volume between regulator 75 andnozzle 57 is much smaller then gas volume in the recirculation loop 60and these two volumes are interconnected. During opening of the valve inregulator 75 the pressure downstream from it and before jet pump nozzle57 is rises rapidly to the regulator's inlet pressure due to thediscrete valve opening and difference (more then about 10 times) betweenvalve cross section flow versus nozzle cross section.

When pressure in the recirculation loop 60 is increased then Pmax isreached, sensed, and the valve of regulator 75 is also closed rapidly.To minimize gas flow throttling on the pressure regulator, its full-opencross section and jet pump nozzle cross section should be calculatedaccordingly.

The invention provides numerous advantages over available fuel cellsystems. For example, advantages of the invention include:

-   -   Increased air feed rate along the cathode working surface, due        to the increasing amount of the air supplied to the each point        provided by recirculation loop 15. This results in better        control of oxidant feed by the air recirculation loop 15 to the        “tri-surface” cathode area. Increased speed leads to increased        active ventilation of cathode pores and surfaces and improved        oxygen supply to the operating cathodes. Implementation of the        oxidant supply design according to the present invention can        increase the rate of oxygen use by the cathode by a factor of        2.5 to 3.5. This increase is equivalent to the increasing the        cathode working pressure by about 1.6-1.9 times.    -   More uniform water distribution and efficient water removal from        the cathode surface. Improved humidification of air entering the        cathode chamber 28 results in improved water concentration        uniformity along the cathode, especially at the gas inlet and        outlet regions. This advantage is primarily due to the mixing of        the air mass flow at higher temperature and lower humidity from        the compressor with the humidified air mass flow at lower        temperature from the recirculation loop, for example in the        proportion of 1:3.    -   This advantage results in a significant reduction in the risk of        fire or explosion in the fuel cell due to the decrease in the        risk of “overdrying” at the inlet section of cathode. It should        also be noted that at a certain level of excess air pressure on        the cathode as compared with the hydrogen pressure on the anode        can result in air leaking onto the anode if the hermetic seal of        the membrane is not maintained. When this occurs, a catalytic        interaction occurs resulting in water formation. Such a        situation does not increase the risk of fire however.    -   More effective water vapor supply to the entire anode surface is        provided. This advantage is due to the continuous circulation of        the humidified hydrogen through the anode chambers.    -   Reduced risk of membrane dehydration thus increasing the        electrochemical performance of the membrane assembly is also        provided. This advantage results because of the anode and/or        cathode active surface limitation.    -   Pulsation of the working (operating) pressure at the three-phase        cathode interface (gas, catalysts and electrolyte) is a        significant advantage, since active ventilation of the pores        occurs and, as a result, nitrogen (as a passive component of        air) is rapidly removed from the active surface of catalysts.        Pressure pulsation in gas-transport pores of the cathode results        in a significant decrease of the “nitrogen cover” effect. This        effect occurs when nitrogen is pressed to the catalysts surface        by the air passing along the three-phase interface through the        gas-transport pores.

Significant advantages under rapid changes in load over a wide range areprovided by the invention. At the same conditions of pressure,temperature and air supply from compressor, the magnitude of the voltagevariations during transit to a new steady state load decreases by afactor of about 1.5 to 2.2.

The pulsating cathode and anode gas feed system of the invention alsoprovides significant advantages for preparing a fuel cell stack forstart-up after a period of storage. Upon shut down, the fuel cellconsumes oxygen fully from air before completely stopping. After longintervals between operation, days or weeks for example, re-start can behindered because the active boundary between cathode and anode is in thestate of nitrogen blockade. That is, access of the components to thethree-phase interface is difficult due to the filling of gas-transportpores (in the cathode, for example) by nitrogen. The pressure pulsationaspect of invention addresses this problem by greatly improving theprocess of starting electrochemical generator after down-time orstorage.

The invention thus significantly increases the reliability and lifetimeof the electrochemical generator. The improvements of this inventionenable the use of PEM fuel cell stacks as electrochemical generators forboth mobile and stationary power units that are able to efficientlyrespond to rapidly cycling load conditions.

While various embodiments of the present invention have been shown anddescribed, it will be apparent to those skilled in the art that manychanges and modifications may be made without departing from theinvention in its broader aspects. The appended claims are thereforeintended to cover all such changes and modifications as fall within thetrue spirit and scope of the invention.

1. A method of operating a PEM fuel cell, which comprises the followingsteps: providing a fuel flow to an anode side of the fuel cell;providing an air flow to a cathode side of the fuel cell; recirculatinga portion of the air flow, after reaction thereof at the anode side,from an output to an input of the cathode side; and selectivelypressurizing the fuel flow and the air flow, with a length of a pulseperiod and a duty cycle of increased pressure within the pulse periodadjusted to an instantaneous power requirement of the fuel cell.
 2. Themethod according to claim 1, which comprises recirculating a portion ofthe fuel flow, after incomplete reaction at the cathode side.
 3. Themethod according to claim 2, which further comprises transferring watergenerated at the cathode side into the recirculated portion of the fuelflow to humidify the fuel flow.
 4. The method according to claim 1,which comprises setting the fuel flow and the air flow as a time-varyingmass flow, the mass flow varying with a load on the fuel cell.
 5. Themethod according to claim 4, wherein the time-varying mass flow isoperative across all loads on the fuel cell.
 6. The method according toclaim 4, wherein the time-varying mass flow comprises discrete pulses.7. The method according to claim 4, which comprises time-synchronizingthe mass flow of the fuel flow with the mass flow of the air flow. 8.The method according to claim 1, which comprises providing the air flowwith a jet pump, and inducing recirculation in the recirculation loopwith the jet pump.
 9. The method according to claim 8, which comprisesfeeding recirculated air from the output on the anode side to a suctioninput of the jet pump, mixing the recirculated air flow portion with afresh air flow portion in the jet pump, and feeding the mixed air flowto the anode side of the fuel cell.
 10. The method according to claim 1,which comprises inducing pressure variations with a pressuresensor-controlled two-position pressure regulator having a first, fullyopen position and a second, fully closed position.
 11. The methodaccording to claim 10, wherein the pressure regulator is a directlycontrolled by hydrogen consumption two-positional pressure regulator inthe fuel feed network, and a slave pressure regulator connected in theair feed network and controlled by the pressure regulator in the fuelfeed network.
 12. A method of operating a PEM fuel cell system, whichcomprises: providing a membrane/electrode assembly (MEA) including aproton exchange membrane (polymer electrolyte membrane, PEM) between ananode chamber with an anode and a cathode chamber with a cathode;supplying fuel to the anode chamber through a hydrogen supply networkconnected to supply hydrogen fuel to the anode; varying a pressure in afeed portion of the hydrogen supply network, under control of a fuelpressure regulator, with a duration of a pressure cycle and a durationof a pressure pulse within the cycle adjusted in dependence on amagnitude of a fuel cell output requirement; supplying air to thecathode chamber through an air supply network connected to supply air tothe cathode; varying a pressure in a feed portion of the air supplynetwork, under control of an air pressure regulator, and synchronizingthe air pressure regulator with the fuel pressure regulator.
 13. Themethod according to claim 12, which comprises measuring a pressure inthe hydrogen supply network in a master measuring chamber of a hydrogensupply pressure regulator, communicating via a feedback line in thehydrogen recirculation loop, and slaving an air supply regulator to thehydrogen supply pressure regulator, for synchronizing the pressurecycles and pulses at the anode with the pressure cycles and pulses atthe cathode.
 14. The method according to claim 12, which comprises:pumping the fuel in the hydrogen supply network with a fuel jet pumphaving an inducing nozzle and a suction input communicating with ananode output of the anode chamber; varying the pressure in the feedportion of the hydrogen supply network with a two-positionpulse-generating hydrogen supply pressure regulator having a hydrogeninput and a hydrogen output communicating with the inducing nozzle ofthe fuel jet pump; selectively setting the regulator to a first, atleast substantially closed position and a second, at least substantiallyopen position for feeding hydrogen to an input of the anode chamber withpulse-fluctuating pressure; and pumping the air with an air jet pumphaving an input receiving air from an air supply and a suction inputcommunicating with a cathode output of the cathode chamber; and settinga pressure in the air supply network with a differential air supplyregulator having an input area communicating with the air supply and anoutput area communicating with an inducing nozzle of the air jet pump.